I’ve spent a lot more time playing with the LTE-M XBee3 so I think I can help a bit better now.
I cannot get it to connect without the solder jumper – it just needs the extra uumph. And the antenna is also definitely not optional.
No matter what I do, it sometimes stops responding. I’m seeing this whether I have it powered off the mayfly or if I have it on a separate Skywire development board, so I don’t think’s a capacitance or brown-out problem. Putting it to sleep and waking it again seems to magically resolve it.
Setting the network technology (“N#) to 2 (LTE-M only) seems to speed the connection up dramatically for me. On waking from sleep it will connect in comfortably under a minute.
And, acting against the manual, I’ve found I can connect even faster and it flakes out less if I put the thing in bypass mode and tell TinyGSM it’s a u-blox (which it is). Go figure.
To put it into bypass mode you set AP5, WR (write changes) and then FR for a force reset. It doesn’t enter bypass until after that.